Microfluidic devices with flexible optically transparent electrodes

ABSTRACT

Microfluidic devices in which electrokinetic mechanisms move droplets of a liquid or particles in a liquid are described. The devices include at least one electrode that is optically transparent and/or flexible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/957,075 filed on Dec. 2, 2015, incorporated herein by reference inits entirety, which is a continuation of U.S. patent application Ser.No. 13/486,714 filed on Jun. 1, 2012, incorporated herein by referencein its entirety, now U.S. Pat. No. 9,227,200 issued on Jan. 5, 2016,incorporated herein by reference in its entirety, which claims priorityto, and the benefit of, U.S. provisional patent application No.61/493,334 filed on Jun. 3, 2011, incorporated herein by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ECCS-0747950,awarded by the National Science Foundation. The Government has certainrights in the invention.

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention pertains generally to microfluidic circuit devices suchas optoelectronic tweezers (OET) devices, and more particularly toelectrode configuration in such devices.

2. Discussion

Optoelectronic tweezers (OET), dielectrophoresis (DEP), electrowetting,opto-electrowetting (OEW), electroosmosis, and electrophoresis devicesare examples of technologies that utilize an electrokinetic mechanism tomove particles, such as polystyrene beads, semiconductor microdisks,nanowires, DNA, cells, and liquids or liquid droplets. OET, for example,is a powerful technology that allows for massive parallel manipulationof single cells with light images at low light intensity. Cells aremanipulated on an OET platform through light induced dielectrophoresis(DEP), a force exerted on a particle subjected to a non-uniform electricfield. OET technology has been used for manipulating many types ofparticles, including polystyrene beads, semiconductor microdisks,nanowires, DNA, and cells. OEW technology has been used for manipulatingliquid droplets on a platform through DEP forces. However, conventionalOET and OEW platforms require a sandwich structures consisting of aphotoconductive electrode and an ITO electrode for applying voltage. TheITO electrode prohibits OET from being integrated with many microfluidiccomponents for conducting more complex multi-step protocols. On theother hand, Au-mesh electrodes have successfully enabled OET and OEWintegration with multilayer PDMS microfluidic devices with valvefunctions. However, Au-mesh electrodes can withstand only a small amountof deformation and could fail after large deformation that inducescracks on the electrode. To realize OET and OEW integrated microfluidicdevices as well as other types of devices that utilize electrokineticmechanisms (e.g., DEP, electrowetting, opto-electrowetting,electrophoresis, or electroosmosis devices) capable of processingcomplex functions or to allow microscopic inspection or fluorescencedetection, the electrode needs to be transparent, conductive, flexible,and/or capable of bonding strongly to OET, OEW, electrowetting, or DEPchips.

BRIEF SUMMARY OF THE INVENTION

Accordingly, we have developed a new fabrication method that allowsembedding of single-walled carbon nanotube thin-film (SWNT) into PDMS,and formation of multiplayer PDMS microfluidic structures with opticallytransparent, electrically conductive, and mechanically deformablemembrane valves. The valve permits repeated deformation without losingits conductivity. Furthermore, the SWNT embedded PDMS membrane can bedeposited deep into the channel without residual carbon nanotubesremaining on the side wall that will affect local electric fielddistribution or on the bonding surface. This allows for a clean surfaceto form strong covalent bonding between OET and PDMS through regularoxygen plasma surface treatment. Our embedded single-walled carbonnanotube thin-film (SWNT) electrodes can be used as a conductive,transparent and flexible layers in poly(dimethylsilane) (PDMS), and ourintegration with optoelectronic tweezers allows for optical manipulationof micro-particles or cells.

In some embodiments, an electrokinetic microfluidic device can include afirst wall, a second wall, a chamber, and a biasing voltage source. Thefirst wall can comprise a transparent electrode. The chamber can beconfigured to hold a liquid and can be between the first wall and thesecond wall. The biasing voltage source can be between the transparentelectrode and the second wall and can create an electric field in thechamber between the transparent electrode and the second wall. Theelectric field can provide an electrokinetic mechanism for moving theliquid or a particle in the liquid in the chamber.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic diagram of a generalized embodiment of amicrofluidic integrated OET device according to the present invention.

FIG. 2 is a schematic diagram of the device shown in FIG. 1 illustratinga SWNT embedded PDMS thin film electrode as the top channel surface andthe deformable check valve according to an embodiment of the presentinvention.

FIG. 3 is a detailed schematic diagram of a microfluidic integrated OETdevice using a SWNT embedded flexible PDMS electrode as the top channelsurface and the deformable check valve according to an embodiment of thepresent invention.

FIG. 4(a)-(d) is a schematic flow diagram of a fabrication process forembedding a thin layer of SWNT network in a multilayer PDMS microfluidicstructure according to an embodiment of the present invention.

FIG. 5: (a) A microscopic image of SWNT embedded valves and channels ina multilayer PDMS microfluidic device. The channel is bonded on an OETchip; (b) SEM image of SWNT/PDMS composite surface; (c)(d) Bright fieldand fluorescence images of 10 μm fluorescence particles underneath aSWNT embedded electrode; (e)(f) Deformed SWNT membrane electrode closesthe bottom channel. The flow channel height is 19 μm; (g) Measured sheetresistance of SWNT embedded PDMS electrode and its relation with opticaltransmittance at 632 nm.

FIG. 6: (a)(b) Transport of a 10-μm particle by light across a membranevalve on an OET integrated multilayer PDMS device. The scale bars areequal to 150 μm; (c) Comparison of particle moving speed in OET usingITO electrode and SWNT embedded PDMS electrode. The light intensity is0.06 W/cm² and the applied frequency is 100 kHz.

FIG. 7 illustrates a light controlled electrokinetic device of which theOET device of FIG. 1 through FIG. 3 is an example.

FIG. 8 and FIG. 9 illustrate additional examples of electrokineticdevices.

DETAILED DESCRIPTION OF THE INVENTION Device Configuration

FIG. 1 through FIG. 3 schematically illustrate the general configurationof an apparatus 100 that incorporates a microfluidic integratedoptoelectronic tweezers (OET) device 102 (which is an example of anelectrokinetic device) according to an embodiment of the presentinvention. In the embodiment shown in FIG. 1 through FIG. 3, the device102 comprises an upper polydimethylsiloxane (PDMS) microfluidic channel106, a bottom photoconductive OET substrate 104, an embedded transparentelectrode 108 in the form of a single-walled nanotube (SWNT) thin-filmelectrode (which is an example of a mesh material), and a bottom channel110. A biasing voltage source 116 (e.g., an alternating current or adirect current voltage source) is electrically coupled to the top andbottom electrodes 104, 108. Unlike a conventional OET device in which afluid chamber is formed between an indium tin oxide (ITO) electrode anda photoconductive substrate, this embodiment of the invention comprisesa multilayer PDMS microfluidic integrated OET with two or more layers ofchannels 106, 110.

In one embodiment, the bottom channel 110 contains aqueous solutionscarrying biological cells or particles 112 and the top channel 106 isused to control the membrane valve 114 (see FIG. 1) formed at the regionwhere the upper channel 106 and bottom channel 110 intersect. In oneembodiment, this elastic membrane 114 works as a mechanical valve thatcan be controlled by pneumatic pressure to close the bottom channel 110and stop the fluid flow. In one embodiment, a peristaltic pump can beachieved by actuating three valves along a channel in series.

Referring more particularly to FIG. 3, in a preferred embodiment amultilayer bottom electrode is provided to establish the photoconductiveOET substrate 104 (which is an example of a multilayer photoconductivewall). In the embodiment illustrated, these layers comprise an ITO layer308, a 50-nm heavily doped n+ hydrogenated amorphous silicon (a-Si:H)layer 306 over the ITO layer, a photoconductive 1-μm undopedhydrogenated amorphous silicon a-Si:H layer 304 over the n+a-Si:H layer,and a 100-nm silicon dioxide layer 302 over the undoped a-Si:H layer.The layer of silicon dioxide 302 is deposited for bonding with PDMS.

In the embodiment illustrated in FIG. 3, an embedded single-wallednanotube (SWNT) thin-film electrode 108 is fabricated on the top surfaceof the bottom PDMS channel 110. In this embodiment, electrode 108comprises a composite SWNT 316/PDMS 314 thin film that establishes thetop surface of the upper channel 110. Because SWNT/PDMS composite thinfilm exists only on the top surface of the channel 110, the surface forbonding with the OET substrate 104 is clean. This allows forming strongcovalent bond with the silicon dioxide 302 coated OET substrate 104.Flow channel 318 is formed at the intersection of the upper 106 andbottom 110 channels.

For OET operation, an alternating current (AC) bias 116 is applied tothe top SWNT electrode 108 and the bottom photoconductive electrode ofthe OET substrate 104. In the embodiment illustrated in FIG. 3, aDigital Micromirror Device (DMD) based projector 320 with an optionalmagnifying lens 322 is used to create dynamic optical images 324 andproject it on the surface of the OET substrate 104 to generate virtualelectrodes and induce DEP forces for cell and particle manipulation.

Device Fabrication

FIG. 4 illustrates a fabrication method that allows embedding SWNT thinfilm into PDMS and forming multiplayer PDMS microfluidic structures withoptically transparent, electrically conductive, and mechanicallydeformable membrane valves according to an embodiment of the invention.The valve permits repeated deformation without losing its conductivity.Furthermore, SWNT embedded PDMS membrane can be deposited deep into thechannel without residual carbon nanotubes remain on the side wall thatwill affect local electric field distribution or on the bonding surface,which gives a clean surface to form strong covalent bonding between OETand PDMS through regular oxygen plasma surface treatment.

Example 1

This fabrication method utilizes transfer-printing techniques and softlithography to fabricate a transparent and flexible SWNT electrode onthe top surface of PDMS channels. The process starts from disperse SWNTin DI water to form aqueous-based SWNT. Powders of commercial SWNT(Carbon Solutions, Inc.) are dissolved in 1 wt % sodium dodecyl sulfate(SDS) solution to prepare solution-based SWNT for vacuum filtration.After SWNT solution is sonicated and centrifuged, a porous anodicaluminum oxide (AAO) filter (Anodisc 47, Whatman Inc.) is used to obtaina SWNT network 404. For the PDMS stamp 402, a master mold was made byusing standard photolithography processes with negative photoresist(SU-8 2025, MicroChem corporation) on a silicon wafer. ThenPoly-dimethylsiloxane (Sylgard 184, Dow Corning) at the ratio 10 base:1curing agent is poured onto the master mold and cured. The PDMS stamp402 with patterns of microfluidic channels are treated by trichloro(1H,1H, 2H, 2H-perfluorooctyl)silane (Sigma-Aldrich, Inc.). The treated PDMSstamp 402 is pressed to contact the AAO filter 406 (FIG. 4a ). After thePDMS stamp 402 is removed from the filter 406, the SWNT thin film 404 istransferred onto the PDMS surface. The PDMS stamp 402 with the SWNT thinfilm 404 was used as the mold for the next casting step. Uncured PDMSwith a ratio of 20 base:1 curing agent spin coated on the PDMS mold tocreate a thin PDMS film (FIG. 4b ). Another thick PDMS layer made at theratio of 5 base:1 curing agent is poured on the thin layer. The twolayers are bonded after curing (FIG. 4c ). The cured PDMS 408 can bepeeled off from the mold 402 and the SWNT thin film 404 is transferredand embedded into the PDMS channels of the cured PDMS 408 (FIG. 4d ).

Example 2

A vacuum filtration method is used to get uniform SWNT thin films. Thesuspension of SWNT is filtered through a filtration membrane to form athin layer of SWNT network. The highly dense SWNT suspension was made byhigh purity (>90%) arc discharge nanotubes from Carbon Solutions, Inc.SWNT were dissolved in 1 wt % sodium dodecyl sulfate (SDS) solution toprepare solution-based SWNT. This highly concentrated SWNT suspensionwas ultrasonically agitated using a probe sonicator for ˜10 minutes. Toremove the carbon particles and impurities, the suspension wascentrifuged at 14000 rpm for 30 min. A porous anodic aluminum oxide(AAO) filter (Anodisc 47, Whatman Inc.) was used in vacuum filtration.The suspension flows through the pores and leaves a thin film of a SWNTnetwork on the surface of the AAO filter. The concentration and thevolume of the flow suspension can control the density of SWNT network.

A PDMS stamp 402 was then used to transfer the SWNT thin film 404 fromthe filter 406. To fabricate the PDMS stamp 402, a master mold was madeby using standard photolithography using a negative photoresist (SU-82025, MicroChem corporation) on a silicon wafer. Poly-dimethylsiloxane(Sylgard 184, Dow Corning) with a ratio of 10 base:1 curing agent waspoured onto the master mold and cured at 65° C. for 4 hr. A PDMS stamp492 with a pattern of microfluidic channels could be peeled off from theSi wafer. Before the stamp contact the filtration membrane, it wastreated by trichloro(1H, 1H, 2H, 2H-perfluorooctyl)silane(Sigma-Aldrich, Inc.), a chemical release agent that lowers the adhesiveforces between the PDMS stamp 402 and the SWNT thin film 404. Thetreated PDMS stamp 402 was then pressed in contact with the SWNT network404 on the AAO filter 406 (FIG. 4a ). After the PDMS stamp 402 isremoved, the SWNT thin film 404 is transferred onto the extrudingsurface of the PDMS stamp 402. This PDMS stamp 402 with a SWNT thin film404 was then used as the mold for casting microfluidic channels.

To cast the microfluidic channels, an uncured PDMS precursor at theratio of 10 base:1 curing agent was poured onto the PDMS stamp 402, andbaked in an oven. The uncured gel-like PDMS precursor infiltrates thevacant regions in the SWNT thin film 404 during the curing process. Thecured microfluidic channels 408 were then be peeled off from the PDMSstamp 402 with a SWNT thin film 404 embedded near the top surface ofchannels. This fabrication technique can be used to embed SWNT thin filmon multilayer PDMS microfluidic devices. To achieve that, the uncuredPDMS precursor at a ratio of 20 base:1 curing agent was spin-coated onthe PDMS stamp 402 to create a thin membrane (FIG. 4b ). A top thickPDMS layer made with a ratio of 5 base:1 curing agent is pressed tocontact the thin PDMS layer. These two layers are bonded after cured(FIG. 4c ). The cured PDMS 408 with a top control channel and a thinfilm SWNT 404 embedded bottom PDMS channel can be peeled off from thefirst PDMS stamp 402 as shown in FIG. 4d . This multilayer PDMS channels408 with an embedded SWNT thin film electrode 404 on the top surface ofbottom channels including the membrane valve locations was then bondedwith an OET device through oxygen plasma treatment.

Note that, in the examples above, the PDMS stamp 402 is not only used asthe mold for casting other microfluidic channels but also transferring aSWNT thin film 404 to the top surface of a channel. This fabricationprocess is compatible with the standard soft lithography process. Sincethe SWNT electrode exists only on the top surface of the flow channels,this provides for a clean surface for forming strong covalent bondingbetween PDMS and silicon dioxide surface of an OET device. The electrodeis continuous, optically transparent, electrically conductive and hasrobust mechanical flexibility allowing large deformation withoutfailure.

Demonstration of SWNT Electrodes Embedded in PDMS

FIG. 5a shows the microscopic image of the SWNT electrode embedded in amultiplayer PDMS microfluidic device. This device has two layers ofmicrofluidic channels. The ring-shape pattern is the bottom channel withSWNT electrodes embedded for sample delivery and cell manipulation. Thetop channel is used to deliver pressure to deform the membrane valvesfor closing a microfluidic chamber or pumping fluid in the bottomchannel. FIG. 5b is the SEM image of a SWNT/PDMS composite electrodeshowing that the randomly distributed SWNT network is completely fixedin the cured PDMS matrix.

Because the a-Si layer on the microfluidic integrated OET isnon-transparent to short wavelength visible light, fluorescence analysishas to come from the top SWNT/PDMS electrode, which one of the reasonthat this electrode needs to be transparent. FIG. 5c and FIG. 5d are thebright field and the fluorescence images of 10-μm fluorescent particlesunderneath the SWNT electrode. Three particles marked by arrows arelocated at the intersection of the top and the bottom channels separatedby a ˜50 μm thick SWNT/PDMS membrane. Comparison between the brightfield and the fluorescence images shows fluorescent analysis isachievable via SWNT electrodes.

The transmission measurement is conducted by using a HeNe laser(wavelength=633 nm, power=2 mW, THORLAB). The relation between themeasured sheet resistance and transmittance of embedded SWNT thin filmsare plotted in FIG. 5g . The transparency and the sheet resistance canbe controlled by the thickness of the SWNT network, while the thicknesscan be adjusted by tuning the amount of SWNT dispersed in solution.

To evaluate the resistance change of the SWNT electrode afterdeformation, the SWNT/PDMS membrane valve was deformed by pressurizingthe control channel repeatedly (FIG. 5e and FIG. 5f ). The valve can bepushed down and closed by applying a pressure of 25 psi in the topcontrol channels while the pressure in the bottom flow channel is 2 psi.The area of the valve is 300 μm×300 μm and the thickness of membrane is˜50 μm. The channel height of the bottom channel is 19 μm. The SWNT/PDMSmembrane electrodes are still conductive after deforming for 30 times.This demonstrates that the SWNT/PDMS electrode is ideal for integratingOET with multilayer PDMS based microfluidic devices to not only supplyelectrical signals but also provide valve and pump functions that arerequired for conducting multi-step and complex protocols.

OET Manipulation

To verify the microfluidic integrated OET platform, we coupled a DMDbased projector (BenQ, MP730) with an inverted microscope. Theprojection lenses of the projector are removed and replaced by a 4×objective lens to focus the light images on the photoconductive surface.10-μm particles were firstly suspended in PBS buffer (conductivity=186μS/cm) and flowed into the microchannels using syringe pumps. Themicrofluidic integrated OET was connected to a function generator(AGILENT 33220A) for supplying sinusoidal ac bias (100 kHz, 10Vpp).Virtual electrodes were turned on by a projected light. Particlesexperienced negative DEP forces and repelled away from the light spots.FIG. 6a and FIG. 6b are the consecutive images showing a 10-μm particleis transported across a deformable PDMS membrane valve by the projectionlight.

The performance of using a SWNT PDMS electrode for OET manipulation hasbeen compared with a regular ITO electrode. The tests were under theapplication of various voltages at 100 kHz. The results are shown inFIG. 6c . This experiment is conducted on an inverted microscope and thelight beam for OET manipulation pass through the SWNT/PDMS electrodebefore illuminating on the photoconductive surface. The SWNT/PDMSelectrode absorbs part of the light, and only 60% of light istransmitted. This contributes to the slightly lower particle speed ofOET manipulation using the SWNT/PDMS electrode.

CONCLUSION

We have successfully demonstrated a novel manufacturing method tofabricate an enabling SWNT/PDMS electrode that is transparent, flexible,deformable, and able to integrate OET with multilayer PDMS microfluidicdevice. The method can transfer SWNT thin film onto the top surfaces ofPDMS channels without leaving any residuals on the sidewall that couldaffect the local electric field distribution and on the bonding surface,which leaves a clean PDMS interface to forming strong covalent bondingwith the silicon dioxide coated OET surface for tight sealing. SWNT/PDMSelectrodes with sheet resistance between 350˜550 ohm and opticaltransmittance of 55% to 80% have been successfully fabricated. OETmanipulation on an integrated PDMS based multilayer microfluidic devicehas also been tested and compared with conventional ITO electrodes. Avalve closing function using SWNT/PDMS membrane has also been shown, andno degradation of electrical conductivity was observed of after repeateddeformation for 30 times. The results show that SWNT electrodes solvethe cracking problems of prior Au mesh electrodes. The microfluidicintegrated OET has potential applications for high throughput, multistepcell-based analysis in the future.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. For example, although flexible, optically transparentelectrodes 108 are illustrated and discussed in the examples above ascomprising SWNT material and being used in an OET device, the flexible,optically transparent electrodes 108 of the present invention cancomprise other materials and can be used in other types of devices.

For example, the flexible, optically transparent electrode 108 of thepresent invention can alternatively comprise materials such asmulti-walled nanotubes. As another example, the electrode 108 cancomprise other types of nanomaterials such as electrically conductive(e.g., metal) nanowires or clusters or electrically conductive (e.g.,metal) nanoparticles or a combination of the foregoing (e.g., acombination of nanowires and nanoparticles). As yet another example, theelectrode 108 can comprise a flexible, optically transparent materialsuch as a polymer or a silicone with embedded nanomaterials such aselectrically conductive (e.g., metal) nanowires or clusters orelectrically conductive (e.g., metal) nanoparticles or a combination ofthe foregoing (e.g., a combination of nanowires and nanoparticles).Thus, the electrode 108 discussed above as comprising an SWNT materialcan alternatively be an electrode that comprises any of the forgoingmaterials (all of which are examples of a mesh material).

Examples of other types of devices that can include the flexible,optically transparent electrode 108 of the present invention includedevices that use an electrokinetic mechanism to move a liquid (e.g., adroplet of the liquid) and/or a particle (e.g., objects, cells, or thelike) in a liquid disposed between electrically biased walls. In suchdevices, an electrode in one or both of two such electrically biasedwalls can be a flexible FIG. 7 through FIG. 9 illustrate examples ofsuch devices.

FIG. 7 illustrates a light-induced microfluidic electrokinetic device700 that can comprise a chamber 710 disposed between a first wall 702comprising a transparent electrode 108 and a second wall 704 comprisinga photoconductive layer 708. The chamber 710 can be sufficientlyenclosed to hold a liquid. As shown, a biasing voltage source 712 (e.g.,an alternating current or a direct current voltage source) can beapplied to the electrode 108 and the second wall 704 (e.g., thephotoconductive layer 708). The voltage source 712 can create anelectric field in the chamber 710 between the first wall 702 and thesecond wall 704.

In some embodiments, the device 700 can be an OET device. Generally inaccordance with the discussion of OET above, light directed onto thephotoconductive layer 708 can selectively create virtual electrodes atthe photoconductive layer 708 that induce DEP forces that move particlesin a liquid in the chamber 710. This is an example of an electrokineticmechanism for moving particles in the liquid in the chamber 710. A lightsource 716 can provide patterns of light 714 through the transparentelectrode 108 to the photoconductive layer 708 to selectively createsuch virtual electrodes.

In other embodiments, the device 700 can be an OEW device. The secondwall 704 can comprise, in addition to the photoconductive layer 708, anoptoelectronic wetting surface (not shown), which can have a hydrophobiccoating (not shown), that faces into the chamber 710. The light pattern714 projected onto the photoconductive layer 708 can selectively createvirtual electrodes at the photoconductive layer 708 that induceelectrowetting forces that move a liquid (e.g., a droplet of the liquid)on the optoelectronic wetting surface (not shown) in the chamber 710.This is an example of an electrokinetic mechanism for moving the liquid(e.g., a droplet of the liquid) in the chamber 710.

FIG. 8 illustrates a microfluidic DEP device 800 (which is an example ofan electrokinetic device) that can comprise a chamber 812 between afirst wall 802 comprising a transparent electrode 108 and a second wall804 comprising a substrate 808 and an array of fixed electrodes 810. Thechamber 812 can be sufficiently enclosed to hold a liquid. A biasingvoltage source 814 can be applied to the transparent electrode 108 andselectively applied to one or more of the electrodes 810 in theelectrode array on the substrate 808. By selectively connecting anddisconnecting the biasing voltage source 814 to and from different onesof the electrodes 810, electric fields, which induce DEP forces, can becreated in the chamber between selected ones of the electrodes 810 andthe transparent electrode 108. The DEP forces can move particles in aliquid in the chamber 812. For example, by selectively connecting andthen disconnected a sequence of the electrodes 810 to the biasingvoltage source 814, a particle in a liquid in the chamber 812 can bemoved from one electrode 810 to another electrode 810 and thus moved inthe chamber 812. This is an example of an electrokinetic mechanism formoving particles in the liquid in the chamber 812.

Because the electrode 108 is transparent, movement of particles in thechamber 808 can be observed. For example, optical monitoring equipment(e.g., a camera) (not shown) can capture through the transparentelectrode 108 images of particles in the chamber 812.

FIG. 9 illustrates a microfluidic electrowetting device 900 (which is anexample of an electrokinetic device) that can be similar to device 800except that device 900 can include an insulating material 902 disposedon the substrate 804 and over the fixed electrodes 810. The material 902can have a surface 904, which can have a hydrophobic coating (notshown). As shown, the insulating material 902 can cover the electrodes810. The electrowetting device 900 can operate generally like the device800 except the electrowetting device 900 can move a liquid (e.g.,droplets of the liquid) in the chamber 812. For example, by selectivelyconnecting and disconnecting the biasing voltage source 814 to differentones of the fixed electrodes 810, electric fields, which induceelectrowetting forces, can be selectively created in the chamber 812between selected ones of the fixed electrodes 806 and the transparentelectrode 108. The electrowetting forces can move the liquid (e.g.,droplets of the liquid) on the surface 904 in the chamber 812 from onefixed electrode 810 to another electrode 810. This is an example of anelectrokinetic mechanism for moving the liquid (e.g., droplets of theliquid) in the chamber 812, and the surface 904 is an example of anoptoelectronic wetting surface.

It is noted that that the devices illustrated in FIG. 1 through FIG. 3and FIG. 7 through FIG. 9 are examples only and can, for example,include additional and/or different elements, and the elements shown canbe arranged in different configurations. Also, those devices can be indifferent orientations. For example the devices shown in FIG. 1 throughFIG. 3 and FIG. 7 through FIG. 9 can be rotated (e.g., can be upsidedown) in alternative configurations.

From the discussion above it will be appreciated that the invention canbe embodied in various ways, including but not limited to the following:

1. A microfluidic integrated OET apparatus, comprising: an upper PDMSchamber with an embedded SWNT thin film electrode; and a lowerphotoconductive OET surface, the photoconductive OET providing a lowerelectrode.

2. The apparatus of embodiment 1, wherein the lower electrode comprises:an ITO layer; a 50-nm heavily doped n+ hydrogenated amorphous silicon(n+a-Si:H) layer over the ITO layer; a photoconductive 1-μm undopedhydrogenated amorphous silicon (a-Si:H) layer over the n+a-Si:H layer;and a 100-nm silicon dioxide layer over the undoped a-Si:H layer.

3. The apparatus of embodiment 2, wherein the silicon dioxide layerfacilitates bonding with PDMS.

4. The apparatus of embodiment 1: wherein the PDMS chamber has a bottomPDMS channel; wherein the bottom channel has a top surface; and whereinthe SWNT embedded PDMS thin film electrode is fabricated on the topsurface of the bottom PDMS channel.

5. The apparatus of embodiment 2: wherein for operating this device, anac bias is applied to the embedded SWNT thin film electrode and thelower OET photoconductive electrode; wherein when light beams illuminatethe a-Si:H layer, virtual electrodes are turned on to create non-uniformelectric field between the lower light-patterned virtual electrode andembedded SWNT thin film electrode for DEP manipulation; and wherein whena virtual electrode is turned on by a projected light beam, cells orparticles experiencing DEP forces are moved away or attracted to thelight pattern.

6. A method for fabricating a PDMS microfluidic channel with an embeddedSWNT thin film electrode, comprising: preparing a SWNT solution;collecting an SWNT network from said solution with an AAO filter;preparing a PDMS stamp as a mold for a microfluidic chamber, said PDMSstamp having an upper mold surface; contacting the AAO filter with theupper mold surface of the stamp, wherein the SWNT network is transferredonto the upper mold surface; spin coating a thin layer of PDMS over theSWNT network; forming a thicker layer of PDMS over the thin layer,wherein the thin and thicker layers bond after curing and form a moldedstructure with a PDMS microfluidic channel; and removing the moldedstructure from the mold; wherein the SWNT network is transferred andembedded into the PDMS microfluidic channel.

7. An electrokinetic microfluidic device, comprising: a first wallcomprising a transparent electrode; a second wall; a chamber betweensaid first wall and said second wall, said chamber configured to hold aliquid; and a biasing voltage source between said transparent electrodeand said second wall for creating an electric field in said chamberbetween said transparent electrode and said second wall for providing anelectrokinetic mechanism for moving said liquid or a particle in saidliquid in said chamber.

8. The device of embodiment 7: wherein said chamber comprises a flexiblemesh material disposed on said second wall; and wherein said transparentelectrode is flexible and embedded in said flexible material.

9. The device of embodiment 8, wherein said chamber is sufficientlyflexible to be pinched closed.

10. The device of embodiment 9: wherein said transparent electrode issufficiently flexible to flex with said chamber as said chamber ispinched closed; and wherein said transparent electrode remainselectrically conductive as said chamber is pinched closed.

11. The device of embodiment 8: wherein said second wall comprises aphotoconductive layer disposed on a conductive layer; and wherein saidbiasing voltage is connected to said conductive layer.

12. The device of embodiment 8, wherein said flexible material comprisesa PDMS material.

13. The device of embodiment 12, wherein said second wall furthercomprises an outer material that facilitates bonding with said PDMSmaterial.

14. The device of embodiment 8, wherein said transparent electrode is athin film, mesh electrode.

15. The device of embodiment 8, wherein said transparent electrodecomprises nanoparticles.

16. The device of embodiment 15, wherein said nanoparticles comprisenanotubes or nanowires.

17. The device of embodiment 7, wherein said second wall comprises aphotoconductive layer responsive to light passed through saidtransparent electrode to create virtual electrodes that induce DEPforces sufficient to move a particle in said liquid.

18. The device of embodiment 7, wherein said second wall comprises: aphotoconductive layer; and an optoelectronic wetting surface that is aninner surface of said chamber.

19. The device of embodiment 18, wherein said photoconductive layer ofsaid second wall is responsive to light passed through said transparentelectrode to create virtual electrodes that induce DEP forces that movea droplet of said liquid in said chamber.

20. The device of embodiment 7, wherein said second wall comprises anarray of fixed electrodes each selectively connectable to said biasingvoltage source.

21. The device of embodiment 20, wherein said second wall furthercomprises an insulating material covering said fixed electrodes.

22. The device of embodiment 7, wherein said transparent electrode is athin film, mesh electrode.

23. The device of embodiment 7, wherein said transparent electrodecomprises nanoparticles.

24. The device of embodiment 23, wherein said nanoparticles comprisenanotubes or nanowires.

25. The device of embodiment 7, wherein said transparent electrodecomprises clusters of nanoparticles.

26. The device of embodiment 7, wherein said transparent electrodecomprises nanoparticles embedded in a flexible material.

27. The device of embodiment 26, wherein said flexible material is apolymer or a silicone.

28. The device of embodiment 7, wherein said transparent electrodecomprises a combination of nanowires and nanoparticles embedded in aflexible material.

Therefore, it will be appreciated that the scope of the presentinvention fully encompasses other embodiments which may become obviousto those skilled in the art, and that the scope of the present inventionis accordingly to be limited by nothing other than the appended claims,in which reference to an element in the singular is not intended to mean“one and only one” unless explicitly so stated, but rather “one ormore.” All structural, chemical, and functional equivalents to theelements of the above-described preferred embodiment that are known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the present claims.Moreover, it is not necessary for a device or method to address each andevery problem sought to be solved by the present invention, for it to beencompassed by the present claims. Furthermore, no element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. An electrokinetic microfluidic device,comprising: (a) a bottom channel configured to hold a liquid,comprising: (i) a first wall comprising a transparent flexible meshelectrode, wherein the first wall is configured to repeatedly deformwithout said mesh electrode losing conductivity after deforming 30 ormore times, wherein the deformation closes off the bottom channelbeneath the mesh electrode; and (ii) a second wall opposite said firstwall comprising an electrode; (iii) wherein said first wall defines anupper surface and said second wall defines a lower surface of at least aportion of said bottom channel; and (b) a top channel overlying at leasta region of said bottom channel, wherein said top channel is formed witha flexible material; and (c) wherein a membrane valve is formed at saidregion where said top channel and said bottom channel overlap; and (d)wherein said membrane valve closes when a pressure of greater than orequal to 25 psi is applied to the top channel.
 2. The device of claim 1,wherein said device is a dielectrophoresis (DEP) device.
 3. The deviceof claim 2, wherein said second wall comprises an array of fixedelectrodes.
 4. The device of claim 1, wherein said second wall of saidbottom channel comprises a photoconductive layer.
 5. The device of claim4, wherein said device is an optoelectronic tweezers (OET) device. 6.The device of claim 1, wherein said photoconductive layer of said secondwall of said bottom channel comprises a hydrophobic coating on saidlower surface of said portion of said bottom channel.
 7. The device ofclaim 6, wherein said device is an optoelectronic wetting (OEW) device.8. The device of claim 1, wherein said flexible material forming saidtop channel is transparent.
 9. The device of claim 8, wherein saidflexible material is a polymer or silicone.
 10. The device of claim 9,wherein said flexible material is polydimethylsiloxane (PDMS).
 11. Thedevice of claim 1, wherein said transparent flexible mesh electrode is athin film.
 12. The device of claim 1, wherein said transparent flexiblemesh electrode comprises nanoparticles.
 13. The device of claim 12,wherein said nanoparticles comprise nanotubes or nanowires.
 14. Thedevice of claim 1, wherein said transparent flexible mesh electrodecomprises clusters of nanoparticles.
 15. The device of claim 1, whereinsaid transparent flexible mesh electrode comprises a single-wallednanotube material embedded within polydimethylsiloxane.
 16. The deviceof claim 1, further comprising a biasing voltage source disposed betweensaid transparent flexible mesh electrode and said second wall.
 17. Thedevice of claim 1, wherein said mesh electrode is not an Au meshelectrode.
 18. A method of manipulating a droplet of a liquid orparticles in a liquid in a microfluidic device comprising a bottomchannel and a top channel, comprising: (a) introducing a liquid orparticles in a liquid to said bottom channel of said microfluidicdevice, said bottom channel comprising an upper wall comprising atransparent flexible mesh electrode configured to repeatedly deformwithout said mesh electrode losing its conductivity; (b) applying anelectrokinetic force to said liquid or said particles in said liquid insaid bottom channel; and (c) closing a membrane valve formed at a regionof overlap between said bottom channel and said top channel by applyingpressure to the top channel; wherein said membrane valve closes when apressure of greater than or equal to 25 psi is applied to the topchannel.
 19. The method of claim 18, wherein said step of applying saidelectrokinetic force to said liquid or said particles in a liquidcomprises applying a biasing voltage between said transparent flexiblemesh electrode embedded in said first wall defining said upper surfaceof said bottom channel and an electrode in a second wall defining alower surface of said bottom channel.
 20. The method of claim 19,wherein said step of applying said electrokinetic force to saidparticles in said liquid further comprises projecting light onto aphotoconductive layer of said second wall, thereby inducing adielectrophoresis force upon said particles in said liquid in saidbottom channel.
 21. The method of claim 19, wherein said step ofapplying said electrokinetic force to said liquid further comprisesprojecting light onto a photoconductive layer of said second wall,thereby inducing an electrowetting force upon said liquid in said bottomchannel.
 22. The method of claim 19, wherein applying pressure to saidtop channel expands said top channel and deforms said first wall of saidbottom channel.
 23. The method of claim 18, further comprising detectingsaid particles in said liquid in said bottom channel.
 24. The method ofclaim 23, wherein-detecting said particles comprises observingfluorescence from said particles in said liquid.
 25. The method of claim23, wherein detecting said particles is performed by observing saidparticles through said.
 26. A method of moving particles in a liquid ina microfluidic device, comprising: (a) introducing particles in a liquidto a bottom channel of said microfluidic device, wherein saidmicrofluidic device comprises: (i) a first wall comprising a transparentflexible mesh electrode, said first wall configured to repeatedly deformwithout said mesh electrode losing its conductivity, and a second wallopposite said first wall comprises an electrode, wherein said first walldefines an upper surface and said second wall defines a lower surface ofat least a portion of said bottom channel; and (ii) a first top channel,a second top channel, and a third top channel overlying a respectivefirst region, a second region, and a third region of said bottomchannel, wherein said first, second, and third top channels are formedwith a flexible material, and further wherein a first membrane valve, asecond membrane valve, and a third membrane valve are formed at saidrespective first, second and third regions; and (b) applying pressureconsecutively to said first, second, and third membrane valves, therebymoving said particles in said liquid in said bottom channel; (c) whereina said membrane valve closes when a pressure of greater than or equal to25 psi is applied to a said top channel forming said membrane valve. 27.The method of claim 26, further comprising applying an electrokineticforce to said particles in said liquid in said channel.
 28. The methodof claim 27, wherein said electrokinetic force is a dielectrophoreticforce or an electrowetting force.
 29. The method of claim 27, furthercomprising detecting said particles in said bottom channel by observingsaid particles through said transparent flexible mesh electrode.
 30. Themethod of claim 29, wherein detecting said particles comprises observingfluorescence from said particles in said liquid.
 31. A method of movingparticles in a liquid in a microfluidic device, comprising: introducinga liquid containing a particle into a bottom channel of saidmicrofluidic device, wherein said bottom channel comprises: an upperwall comprising a transparent flexible mesh electrode, said upper wallconfigured to repeatedly deform without said mesh electrode losing itsconductivity; and a lower wall comprising an electrode; a top channeloverlying a portion of said bottom channel, said top channel formed witha flexible material; and a membrane valve formed where said bottom andtop channels overlap; and applying pressure to said membrane valve tomove said particle in said liquid in the bottom channel; wherein saidmembrane valve closes when a pressure of greater than or equal to 25 psiis applied to the top channel.
 32. The method of claim 31, furthercomprising applying an electrokinetic force to said particles in saidliquid in said channel.
 33. The method of claim 31, wherein saidelectrokinetic force is a dielectrophoretic force or an electrowettingforce.
 34. The method of claim 31, further comprising detecting saidparticles in said bottom channel by observing said particles throughsaid transparent flexible mesh electrode.
 35. The method of claim 34,wherein detecting said particles comprises observing fluorescence fromsaid particles in said liquid.
 36. The method of claim 31, wherein thebottom channel height is about 19 microns.